Charging and discharging batteries is a chemical reaction, but 18650 lithium battery is claimed to become the exception. Battery scientists discuss energies flowing out and in of the battery as an element of ion movement between anode and cathode. This claim carries merits but if the scientists were totally right, then this battery would live forever. They blame capacity fade on ions getting trapped, but as with all battery systems, internal corrosion and other degenerative effects also known as parasitic reactions around the electrolyte and electrodes till be a factor. (See BU-808b: The causes of Li-ion to die?.)
The Li ion charger is really a voltage-limiting device which has similarities to the lead acid system. The differences with Li-ion lie in a higher voltage per cell, tighter voltage tolerances and the absence of trickle or float charge at full charge. While lead acid offers some flexibility with regards to voltage stop, manufacturers of Li-ion cells are incredibly strict in the correct setting because Li-ion cannot accept overcharge. The so-called miracle charger that offers to prolong life of the battery and gain extra capacity with pulses and other gimmicks fails to exist. Li-ion is actually a “clean” system and only takes just what it can absorb.
Li-ion using the traditional cathode materials of cobalt, nickel, manganese and aluminum typically charge to 4.20V/cell. The tolerance is /-50mV/cell. Some nickel-based varieties charge to 4.10V/cell; high capacity Li-ion may go to 4.30V/cell and higher. Boosting the voltage increases capacity, but going beyond specification stresses battery and compromises safety. Protection circuits built in the pack do not allow exceeding the set voltage.
Figure 1 shows the voltage and current signature as lithium-ion passes throughout the stages for constant current and topping charge. Full charge is reached when the current decreases to between 3 and 5 percent from the Ah rating.
The advised charge rate of any Energy Cell is between .5C and 1C; the whole charge time is approximately 2-three hours. Manufacturers of these cells recommend charging at .8C or less to extend battery life; however, most Power Cells may take a higher charge C-rate with little stress. Charge efficiency is all about 99 percent along with the cell remains cool during charge.
Some Li-ion packs may go through a temperature rise around 5ºC (9ºF) when reaching full charge. This might be due to the protection circuit and/or elevated internal resistance. Discontinue utilizing the battery or charger in case the temperature rises over 10ºC (18ºF) under moderate charging speeds.
Full charge takes place when the battery reaches the voltage threshold along with the current drops to 3 percent from the rated current. Battery power can also be considered fully charged if the current levels off and cannot decline further. Elevated self-discharge could be the cause of this disorder.
Boosting the charge current will not hasten the complete-charge state by much. Even though the battery reaches the voltage peak quicker, the saturation charge will take longer accordingly. With higher current, Stage 1 is shorter however the saturation during Stage 2 is going to take longer. A high current charge will, however, quickly fill the battery to around 70 percent.
Li-ion is not going to must be fully charged as is the situation with lead acid, nor will it be desirable to do so. The truth is, it is advisable never to fully charge as a high voltage stresses battery. Choosing a lower voltage threshold or eliminating the saturation charge altogether, prolongs battery lifespan but this reduces the runtime. Chargers for consumer products go for maximum capacity and can not be adjusted; extended service life is perceived less important.
Some lower-cost consumer chargers can make use of the simplified “charge-and-run” method that charges a lithium-ion battery in just one hour or less without visiting the Stage 2 saturation charge. “Ready” appears if the battery reaches the voltage threshold at Stage 1. State-of-charge (SoC) at this stage is around 85 percent, a level that may be sufficient for several users.
Certain industrial chargers set the charge voltage threshold lower on purpose to extend life of the battery. Table 2 illustrates the estimated capacities when charged to various voltage thresholds with and without saturation charge. (See also BU-808: The way to Prolong Lithium-based Batteries.)
As soon as the battery is first wear charge, the voltage shoots up quickly. This behavior can be in comparison to lifting a weight by using a rubber band, creating a lag. The capacity will eventually get caught up once the battery is practically fully charged (Figure 3). This charge characteristic is typical of all the batteries. The higher the charge current is, the greater the rubber-band effect is going to be. Cold temperatures or charging a cell with high internal resistance amplifies the outcome.
Estimating SoC by reading the voltage of a charging battery is impractical; measuring the open circuit voltage (OCV) following the battery has rested for a couple hours is actually a better indicator. As with most batteries, temperature affects the OCV, so does the active material of Li-ion. SoC of smartphones, laptops as well as other devices is estimated by coulomb counting. (See BU-903: How to Measure State-of-charge.)
Li-ion cannot absorb overcharge. When fully charged, the charge current should be stop. A continuous trickle charge would cause plating of metallic lithium and compromise safety. To minimize stress, keep your lithium-ion battery with the peak cut-off as short as you can.
When the charge is terminated, battery voltage actually starts to drop. This eases the voltage stress. With time, the open circuit voltage will settle to between 3.70V and 3.90V/cell. Keep in mind that energy storage companies which has received an entirely saturated charge helps keep the voltage elevated for a longer than a single which has not received a saturation charge.
When lithium-ion batteries must be left from the charger for operational readiness, some chargers apply a brief topping charge to compensate for the small self-discharge battery along with its protective circuit consume. The charger may kick in if the open circuit voltage drops to 4.05V/cell and turn off again at 4.20V/cell. Chargers made for operational readiness, or standby mode, often permit the battery voltage drop to 4.00V/cell and recharge to simply 4.05V/cell rather than the full 4.20V/cell. This reduces voltage-related stress and prolongs life of the battery.
Some portable devices sit inside a charge cradle inside the ON position. The present drawn through the device is called the parasitic load and might distort the charge cycle. Battery manufacturers advise against parasitic loads while charging simply because they induce mini-cycles. This cannot continually be avoided as well as a laptop coupled to the AC main is unquestionably an instance. The battery could possibly be charged to 4.20V/cell after which discharged by the device. The anxiety level on the battery is high since the cycles occur with the high-voltage threshold, often also at elevated temperature.
A portable device must be turned off during charge. This permits battery to reach the set voltage threshold and current saturation point unhindered. A parasitic load confuses the charger by depressing battery voltage and preventing the current inside the saturation stage to drop low enough by drawing a leakage current. Battery power might be fully charged, however the prevailing conditions will prompt a continued charge, causing stress.
Whilst the traditional lithium-ion features a nominal cell voltage of 3.60V, Li-phosphate (LiFePO) makes an exception having a nominal cell voltage of three.20V and charging to 3.65V. Somewhat new is the Li-titanate (LTO) using a nominal cell voltage of 2.40V and charging to 2.85V. (See BU-205: Forms of Lithium-ion.)
Chargers of these non cobalt-blended Li-ions will not be appropriate for regular 3.60-volt Li-ion. Provision has to be intended to identify the systems and supply the proper voltage charging. A 3.60-volt lithium battery inside a charger designed for Li-phosphate would not receive sufficient charge; a Li-phosphate within a regular charger would cause overcharge.
Lithium-ion operates safely within the designated operating voltages; however, the battery becomes unstable if inadvertently charged to some beyond specified voltage. Prolonged charging above 4.30V on the Li-ion designed for 4.20V/cell will plate metallic lithium on the anode. The cathode material becomes an oxidizing agent, loses stability and produces carbon dioxide (CO2). The cell pressure rises and when the charge is able to continue, the present interrupt device (CID) in charge of cell safety disconnects at 1,000-1,380kPa (145-200psi). In case the pressure rise further, the safety membrane on some Li-ion bursts open at about 3,450kPa (500psi) and also the cell might eventually vent with flame. (See BU-304b: Making Lithium-ion Safe.)
Venting with flame is linked to elevated temperature. A fully charged battery carries a lower thermal runaway temperature and will vent earlier than the one that is partially charged. All lithium-based batteries are safer at the lower charge, and for this reason authorities will mandate air shipment of Li-ion at 30 percent state-of-charge rather dexkpky82 at full charge. (See BU-704a: Shipping Lithium-based Batteries by Air.).
The threshold for Li-cobalt at full charge is 130-150ºC (266-302ºF); nickel-manganese-cobalt (NMC) is 170-180ºC (338-356ºF) and Li-manganese is all about 250ºC (482ºF). Li-phosphate enjoys similar and temperature stabilities than manganese. (See also BU-304a: Safety Concerns with Li-ion and BU-304b: Making Lithium-ion Safe.)
Lithium-ion is not really the only battery that poses a safety hazard if overcharged. Lead- and nickel-based batteries are also proven to melt down and cause fire if improperly handled. Properly designed charging gear is paramount for many battery systems and temperature sensing can be a reliable watchman.
Charging lithium-ion batteries is simpler than nickel-based systems. The charge circuit is simple; voltage and current limitations are easier to accommodate than analyzing complex voltage signatures, which change because the battery ages. The charge process could be intermittent, and Li-ion will not need saturation as is the case with lead acid. This provides a major advantage for renewable power storage for instance a solar power panel and wind turbine, which cannot always fully charge the 18650 battery pack. The absence of trickle charge further simplifies the charger. Equalizing charger, as they are required with lead acid, is not necessary with Li-ion.